1. Field of the Invention
The present invention relates to a line driver circuit having a protective circuit against excess currents, for protecting a transistor at the output stage from being thermally destroyed by a short circuit in the output circuit.
The line driver circuit is used to transmit signals from, for example, a terminal unit, through a transmission line, to a computer a long distance away, or vice versa. If a short occurs in the load of the line driver circuit, for example, in the transmission line, excess currents flow through transistors which are provided in the output stage, resulting in a thermal destruction of the transistors. The shorts may occur by, for example, a faulty connection of the transmission line to the line driver circuit during an inspection of the circuit.
2. Description of the Prior Art
In a conventional line driver circuit, a protective circuit for protecting the output-stage transistor from being thermally destroyed is provided as will be hereinafter described in detail. However, there are disadvantages in the conventional line driver circuit. The first disadvantage is that the maximum short-circuit current flowing through the output-stage transistor is too large to protect the output-stage transistor as hereinafter described in detail. The second disadvantage is that thhe maximum short-circuit current flowing through the output-stage transistor cannot be precisely determined, because the maximum short-circuit current is determined by resistances and the characteristics of the transistors, both having variations due to the manufacturing processes, as will be hereinafter described in detail.